This invention relates to a porcelain clad current transformer having an expansion chamber filled with insulating oil and gas.
FIG. 1 shows a sectional view of an expansion chamber in a conventional current transformer of this kind as disclosed by Japanese Utility Model Publication No. 28100 of 1965, wherein a primary coil 1 is disposed in an insulating tube 2. The expansion chamber 3 is defined by an upper flange 31, a cylindrical side plate 32 and a base plate 33, and the assembly is secured to the top of the insulating tube via the base plate. A cover 4 is attached over the flange with bolts 6 passing through a packing 5.
Current is supplied to the primary coil 1 through terminals 7 secured through the side plate 32 of the chamber and connected to lead straps 8. Each terminal 7 is mounted on a terminal board 9 outside of the expansion chamber.
Insulating oil 10 for cooling the primary coil fills the insulating tube 2 and most of the expansion chamber 3. The upper portion of the chamber is filled with nitrogen gas 11 to absorb any pressure increase caused by the thermal expansion of the insulating oil. To improve heat dissipation a finned radiator 12 is welded around the side plate 32 of the expansion chamber.
To assemble such a conventional current transformer the primary terminals 7 are first mounted through the side plate 32, and the base plate 33 of the expansion chamber is then bolted onto the insulating tube 2. The lead straps 8 are next connected between the primary coil 1 and the terminals 7, whereafter the cover 4 is bolted onto the flange 31 through the packing 5. Finally, the insulating oil 10 and nitrogen gas 11 are supplied to and sealed within the expansion chamber.
Such a conventional porcelain clad current transformer has some drawbacks, however, as described below. First of all, the lead straps 8 have to be installed by inserting them manually from the relatively remote area of the flange 31, which makes the assembling job difficult and time consuming. Secondly, the inner edge of the packing 5 is always in contact with the nitrogen gas 11, and the resulting deterioration of the packing hampers its leak protection properties. Thirdly, the expansion chamber occupies a relatively large vertical space due to the radiator 12 and the primary terminals 7 both being mounted on its side plate 32. Finally, the side plate has to be made of an expensive non-magnetic material to reduce heat generation caused by structural loss accompanied by penetrating current flow through the side plate.